This invention concerns a compact continuous casting line for the production of thin slabs.
The invention is applied both in casting lines at least partially curved which include, at the outlet of the ingot mould, a straight and vertical solidification area followed by the straightening of an already solidified product, and also to casting lines which include a solidification area at least partially curved, where the straightening can be performed on a product which has not yet completely solidified.
Furthermore, the invention is applied also to casting lines where at least the end segment of the ingot mould is curved in a coherent manner with the curve of the casting line.
The possible furnace to restore the temperature of the product, which is associated with the casting line according to the invention, can be of the tunnel type placed in line with the casting machine and the shearing assembly, or it may be of the type including an inlet and outlet rollerway, and the transverse travel of the segments of thin slab.
The state of the art covers continuous casting plants which comprise at least an ingot mould associated at the lower part with an assembly to extract and straighten the thin slab.
According to whether the area of solidification for the slab emerging from the ingot mould is straight or partly curved, the extraction and straightening assembly acts on a product which is either completely solidified or has only the skin solidified and the core still liquid.
The extraction and straightening assembly is normally associated at the end with one or more, generally two, pinch roll assemblies, followed downstream by a shearing assembly able to shear the slab into single segments which are then rolled or possibly discarded.
In continuous casting lines, at present the segments of slab thus formed are subjected to a descaling operation, normally performed by a rotary device placed between the shearing assembly and the temperature-restoration furnace, if any, placed upstream of the rolling train.
The function of the descaling device upstream of the temperature-restoration furnace is to eliminate the scale which has formed during the casting process, and also any casting powders on the surface of the slab.
This descaling action prevents any hard and resistant scale from forming as the segments of slab pass through the temperature-restoration furnace. It also prevents the formation of compounds of scale and powder, which are difficult to eliminate later, even by an intense descaling action performed at the outlet of the furnace.
Moreover, by performing the descaling action upstream of the temperature-restoration furnace it is possible to obtain on the surface of the slab a residual layer of scale of a uniform thickness and composition which is more easily removed by the normal descaling means placed downstream of the furnace and upstream of the rolling train.
The rotary descaling device normally comprises a plurality of nozzles, advantageously also rotary, which deliver water under high pressure onto the faces of the segments as they pass through.
One problem caused by the presence of this descaling assembly is that this water delivered by the nozzles flows over the upper surface of the segment of slab being descaled, even for long sections and both upstream and downstream of the descaling zone.
The flow of this descaling water causes considerable problems such as the unhomogeneous cooling of the slab, and therefore areas of excessive cooling are created. Moreover, the flow of water causes operational problems and possible damage to the equipment placed upstream and downstream of the descaling device.
To be more exact, if the tunnel furnace is placed in line with the casting machine and the shearing assembly, the entry of water inside the furnace is extremely dangerous, since the creation of steam inside the tunnel can seriously damage the furnace itself.
In order to obviate this problem, at present a pair of rolls are included, and are arranged upstream and downstream of the descaling device and in contact with the upper face of the segment; these rolls are. suitable to confine the water in the descaling zone, and prevent it from flowing outside the said area.
However, the proximity of the descaling device to the furnace, particularly in the case of a tunnel furnace, does not eliminate the risk that, in the event of a malfunction or if the retaining rolls are worn, some water may in any case enter inside the furnace and cause the serious problems mentioned above.
Another problem to be found in these casting lines is that the length of the line itself is increased by the inclusion of the descaling device and the pair of retaining rolls.
It is well-known in the field that even a limited reduction in the spaces occupied by the lines, and a shortening of the machine, is important in economic terms, given the considerable investment required by this type of line.
In plants known to the state of the art, the inclusion of the descaling device causes a lengthening of the line and also causes the temperature-restoration furnace to be farther from the shearing assembly. The greater distance of the furnace from the shearing assembly causes the segment of slab to lose a high level of heat, due to irradiance.
The reduction in the temperature of the segments as they enter the furnace involves a necessary increase in the furnace's capacity to re-heat the segments to the desired rolling temperature.
This increase, however, cannot be pushed beyond certain limits, otherwise it could cause a considerable loss of efficiency, excessive fuel consumption and a reduction in the working life of at least some of the components of the furnace, particularly the rolls.
If the rolls are water cooled, the increase in temperature inside the furnace, to compensate for the heat losses from irradiance, makes it necessary to increase the intensity of cooling, with a consequent increase in consumption and in operating costs.
A reduction in the temperature of the segments of slabs at the inlet to the furnace involves an increase in heating times, which can lead to the necessity of increasing the size of the furnace itself. This causes a further increase in the overall length of the plant, with an increase in the costs of all the structures and the connected plant, for example the sheds, the foundations, the pipes etc.
The line is also further lengthened because it is impossible to bring the descaling device nearer the shearing assembly because of the presence of the device to extract the dummy bar.
This extractor device normally consists of a slide connected to the shearing assembly; it includes a first position for the recovery of the dummy bar where it is in an oblique position at the outlet of the shearing assembly, and a second inoperative position where it is raised and substantially parallel to the rolling line.
The position assumed by the extractor device when it recovers the dummy bar therefore makes it necesary to distance the descaling device from the shearing assembly so as to avoid contact between the descaling device and the extractor device.
This causes a greater bulk in the casting line, a further distancing of the furnace and therefore a great effect on the overall costs.